In the metallization step utilized preparatory to etching of conductors about the outer surfaces of integrated circuits, step-coverage of conductive metal films (typically aluminum or aluminum alloys) is poor over surface discontinuities, such as recesses and contact holes/vias. The problem becomes progressively worse as the dimensions of the integrated circuit components shrink. Poor step coverage is the result of the shadow effect in the deposited film at the sidewalls of the steps or holes, and is illustrated with reference to FIG. 1. A semiconductor wafer fragment 10 is illustrated with a contact opening or via 12. A metallized layer 14 of aluminum or its alloys has been sputtered atop wafer fragment 10. As is apparent, sputtered layer 14 does not completely nor adequately fill contact opening 12, which is an inherent drawback in sputtered metal films.
Poor step coverage can be overcome to some degree by selective chemical vapor deposition (CVD) of tungsten or other elements, by a chemical vapor deposition using high temperature and/or bias sputtering, or by supplemental metal deposition using multiple alternation sequences involving a combination of evaporation and re-sputtering. However, improvements in step coverage from such methods are achieved at the expense of several drawbacks. For example, high temperature and/or bias sputtering results in poor film quality in terms of surface morphology, electromigration lifetime (for bias sputtering) and increased argon incorporation. Regarding CVD of tungsten, the resulting film resistivity is about three times higher than that of aluminum or aluminum alloys. CVD technology has the general drawbacks of high cost, poor surface morphology and film qualities (especially true with aluminum). Alloy material interconnects, especially where the primary constituent is aluminum, are preferred to minimize junction spiking and electron migration which can lead to circuit failure in operation. CVD is described generally in "Silicon Processing For The VLSI Era", (Vol. 2-Process Integration), by S. Wolf, published 1990 by Lattice Press, Sunset Beach, Calif., at Section 4.5., which is hereby incorporated by reference.
Planarization of conductive films is one method of obtaining improved step coverage of sputtered films, as compared to the as-deposited film quality. Such can be conducted with rapid thermal processing (RTP) or with the use of laser energy. Rapid thermal processing typically employs lamps which rapidly heat the metallized surface, causing it to melt. During the molten period, mass transport of the conductive metal occurs which results in flow of the metal completely into the contact holes/vias. As well, the film surface is driven flat due to the high surface tension and low viscosity of molten metals. However, RTP has the drawback of potentially damaging the underlying substrate because of the heating time required to cause the desired flow. This is one reason that laser planarization is preferred over RTP.
Laser planarization achieves the same effect as RTP by impinging laser energy on the metallized surface to cause the melting and flow. The use of continuous or pulse lasers to melt and planarize thin aluminum films to fill high aspect ratio contact holes/vias is an attractive approach to Ultra Large Scale Integration (ULSI) circuit metallization. It is a low thermal budget, simple, and effective technique for planarizing metal layers and filling inter-level contacts at the cost of only one additional step to the standard process flow.
The technique of laser planarization has shown promise in improving step coverage of aluminum alloy films in micron/submicron geometry contacts and contact vias. However, laser planarization of aluminum in particular is not without drawbacks. First, aluminum is a highly reflective metal which reflects approximately 93% of wavelengths in the region down to 200 nm. Accordingly, aluminum reflects a significant amount of laser energy which results in inefficiency.
Second, laser planarization is typically conducted in an evacuated chamber at very low, vacuum-like pressures. The intent is to eliminate oxygen to avoid degradation reactions which would otherwise occur during the process. Typical pressures within the laser planarization chambers are between 10.sup.-9 to 10.sup.-7 atmospheres. Under these conditions, the vaporization/boiling temperatures of aluminum are 1632.degree. C. and 1782.degree. C., respectively. Accordingly, care must be exercised to assure that the applied laser energy will not raise the temperature of the aluminum film above the 1632.degree. C. to 1782.degree. C. vaporization temperature, which would cause the aluminum from the applied surface to ablate from the surface.
On the other hand, the applied energy must be at least sufficient to cause aluminum melting to achieve the desired step coverage/planarization effect. Because of aluminum's high reflectivity, however, this minimum amount of energy is pushed higher as a significant amount is reflected away. Accordingly, the range between the applied energy to avoid ablation and to achieve adequate melting is very narrow. This window range is typically referred to as the "process window". In production, conditions must be maintained within this window to achieve suitable results. The closer the melting temperature and ablation temperature are to one another, the narrower the process window and accordingly the more tedious and difficult production becomes.
Accordingly, neither CVD nor sputtering followed by laser planarization techniques produce the most desired results.